VARIETAL RESISTANCE 279 



selection and propagation, which involves breeding for highly 

 resistant special strains. A good example of selective breeding is 

 afforded by the work done in South Africa with regard to jassid- 

 resistant strains of cottons. Varietal hybridisation is practised 

 with the object of combining desirable features present only in 

 separate and different strains. It is dependent for its success upon 

 the purity of the segregation of the resistance and other factors 

 involved. The method has been practised by Harland in Barbados 

 in the crossing of Sea Island and Upland cottons with resistant 

 native strains as a means of control against the leaf blister mite 

 {Eriophyes gossypi). In the Southern U.S.A. the crossing of 

 commercial sweet maize with native field varieties is stated 

 to have resulted in strains better able to resist attack by the 

 corn earworm (Heliothis ohsoleta). Inter-specific hybridisation 

 has resulted in the production of the Leconte and Keiffer varieties 

 of pears which are resistant to the San Jose scale. These varieties 

 are Fj hybrids between Pyrus communis and the Chinese pear 

 (Pyrus sinensis) ; both inherit the desirable qualities of P. 

 communis^ in combination with the resistance to scale injury 

 characteristic of P. sinensis. The need for discovering varieties 

 of spring oats resistant to infestation by frit-fly is referred to on 

 p. 282, and it appears that more promise of success is likely to 

 result by adopting varietal hybridisation, rather than by relying 

 upon any varieties, yielded by selective breeding, exhibiting the 

 desired combination of characters. Graft hybridisation, which is an 

 old method, involves the grafting of commercially valuable, but 

 highly susceptible, plants upon root-stocks of resistant varieties. 

 It has proved successful in the now classical cases of apple trees 

 infested by woolly aphis, and of vines attacked by the Phylloxera. 



The foregoing remarks will serve as a brief outline of the nature 

 of the problems involved and, in further illustration of this phase 

 of insect control, certain examples will be taken in connection 

 with recent researches. 



1. With regard to the susceptibility of different varieties of 

 apple trees to the attacks of the woolly aphis (Eriosoma lanigera), 

 an extensive literature exists on the subject, but no very con- 

 clusive deductions can be drawn. Some authorities maintain 



